1. Field of the Invention
This invention relates to aircraft marker beacon receiving and indicating systems and, more particularly, to improvements thereof for enhancing safety and convenience during instrument landings by decreasing the distractions and burden of functions heretofore manually performed by the pilot during such critical and busy phase of flight. The improved system provides automatic control over the functionas of reducing receiver sensitivity and muting of the audio output from the receiver, which is desirable and customarily done manually once the general proximity to a beacon has been sensed and indicated and during that period that the aircraft is merely completing its passage over the beacon.
2. Description of the Prior Art
The most relevant known prior art, the problems it leaves unsolved and the distinctions between the present invention and what has heretofore long been the conventional standard practice in connection with the relatively specialized field of technology involved can all be best understood by initially considering a typical aircraft marker beacon receiving and indicating system of the conventional type now in almost universal use and the manner in which it is typically operated during an instrument landing. For that purpose, such a system has been shown in FIG. 1 of the drawings, to which reference will be made as this explanation proceeds.
Attention and appropriate action by the pilot of an aircraft during an instrument landing is required with respect to a number of different instrument outputs, aircraft conditions, weather conditions, air traffic conditions, and the demands of radio communication with an airport control tower; moreover, the pilot must mentally integrate information from a plurality of such sources with great rapidity in order to take the proper actions at the proper times for successfully executing the instrument landing. The relevant instruments to be monitored during the final glide slope phase of an instrument landing include the altimeter, the airspeed indicator, the rate of descent indicator, the engine and other aircraft indicating instruments, the communications radio and various navigational aid instruments for indicating the direction and distance of the aircraft from the runway. Among the latter are the indicator lights and audible signals provided by three ground-based marker beacon radio transmitters disposed at intervals along the glide slope path aligned with a runway. The inner beacon is located at or closely adjacent the end of the runway, and, although the exact distances may vary from airport to airport, the outer beacon is typically located no more than four miles from the inner beacon, with the middle marker therebetween.
Each of the beacons transmits radio signals upwardly in a generally conical pattern. The beacons are sufficiently displaced from each other along the ground and their radiation patterns are sufficiently well defined that their respective signals will be received successfully by an aircraft proceeding along the proper glidepath. The radiation patterns of the beacons are restricted in extent such that an aircraft traveling at glidepath speed, regardless of size and type of engine, will typically enter, traverse and leave the zone of reception for signals from a given beacon within less than 15 seconds. The radiation patterns of the beacons are also stronger in the inner more vertical portion of their conical extent than in the outer portions thereof, so that the signals therefrom will be received with a much greater signal strength by an aircraft which is substantially directly over a beacon than when the aircraft is approaching or leaving the beacon. The signals transmitted by each of the beacons is a radio frequency carrier of about 75 mHz, amplitude modulated by distinctive Morse code pulse groups of an audio frequency which is different for each of the three beacons (400 Hz for the outer beacon, 1300 Hz for the middle beacon and 3000 Hz for the inner beacon). After separation of such modulation signals from the carrier by demodulating detection in the tuner portion of an aircraft receiver, the signals emanating from each beacon are distinguishable both audibly and by electrical filtering.
In general, such marker beacon signals are presented to the pilot of an aircraft in two ways--first, as an audible output on a loudspeaker in the cockpit area (or on earphones worn by the pilot), and secondly, by the illumination of one of three panel lights respectively corresponding to the three beacons (a blue light for the outer beacon, an amber light for the middle beacon and a white light for the inner beacon). As the aircraft proceeds down the glidepath toward the runway, the successive reception and presentations of the signals from each beacon provide the pilot with confirmation that the aircraft is directionally on the glidepath to the runway, but, primarily are used to provide the pilot with an indication of the distance of the aircraft from the end of the runway and a means of assuring that the aircraft is elevationally on the glidepath prescribed for the involved airport (which is defined for each airport in terms of the altitude at which the aircraft should be as it passes over the center of each of the outer and middle markers). Typically, an aircraft is required to be at an altitude of less than 1000 feet over the outer marker, less than 500 feet over the middle marker and in condition ready for touchdown over the inner marker, with the entire traversal of the glidepath between the outer and inner markers usually lasting no more than a minute for commercial jet aircraft.
The nature of the marker beacon signals, the type of information available therefrom and the manner in which such information needs to be utilized inherently create two types of problems.
First, the level of required concentration and activity of the pilot during the final glideslope phase of an instrument landing is so high that any unnecessary distraction or interference with other activities caused by the marker beacon system is intolerable; such distraction and interference would arise from continuance of the audible output over the loudspeaker or pilot's earphones (which may also be needed for communication with the control tower) of the coded tone signals from a marker beacon during the entire traversal of its radiation pattern by an aircraft, whereas such audible signals are really needed only momentarily to alert the pilot to the fact that the aircraft has entered the radiation pattern overlying a particular beacon (the identity of the beacon being traversed thereafter being readily ascertainable from reference to which of the three indicator lights is lit). The practical solution typically followed by pilots for dealing with such first problem is to manually open the switch connecting the marker beacon receiver with the loudspeaker or headphones, once entry into the reception area for a particular beacon has been noted, with the intention of reconnecting the beacon receiver with the audio transducer before the reception area for the next beacon will be reached--unfortunately, the latter restorative step may be overlooked or not feasible because of the press of other activities.
Secondly, presuming that the marker beacon receiver in an aircraft is being operated at a sensitivity to radio frequency signals sufficient to commence receiving the signals from a given beacon as soon as the aircraft enters the outer portion of the radiation pattern of such beacon, no definite indication would be provided to the pilot as to when the aircraft has reached a position substantially directly above the beacon (which is when the check for altitude being at the prescribed level needs to be made). The imperfect solution for such second problem has been for aircraft marker beacon receivers to be provided with a manual switch to be opeated by the pilot, once entry into the reception zone for a given beacon has been indicated, for temporarily descreasing the sensitivity of the receiver, so that the indicator lights (and audible output device, if it has not been turned off) will provide a reliable indication of when the aircraft has proceeded to a position substantially over the central part of the beacon's radiation pattern (at which the radiated signals are strongest)--but this conventional expedient, requiring two manual switch operations by the pilot for each of the outer and middle beacons, is subject to the same shortcomings as noted in connection with intended temporary deactivations of the loudspeaker, and a further operational disadvantage has arisen from the manner in which prior marker beacon systems have implemented such manual control over receiver sensitivity, as hereinafter explained.
Specific reference is now made to FIG. 1 of the drawings, in which a typical prior aircraft marker beacon receiver system is depicted in block diagram form. The marker beacon signal antenna 10 of the aircraft is coupled as at 12 to the signal input terminal of what has been generally denominated as the tuner portion 20 of the receiver system. The tuner 20 is typically of the double superheterodyne type and includes a 75 mHz bandpass filter 22 for receiving radio frequency signals from the antenna 10; a first mixer 24 having an associated oscillator 26 for converting signals passing through the filter 22 to a first intermediate frequency of, say, about 10.7 mHz and delivering the same to a first intermediate frequency amplifier 28; a second mixer 30 having an associated oscillator 32 for converting signals from the output of the amplifier 28 to a lower second intermediate frequency of, say, about 455 kHz and delivering the same to a second intermediate frequency amplifier 34; a demodulating detector 36 for receiving the output from the amplifier 34 and removing the intermeidate frequency carrier component therefrom to present an audio frequency output corresponding to the coded tone group component of the modulated radio frequency signals received from a ground-based marker beacon transmitter; and an automatic gain control circuit 38 including a rectifier 39 for feeding a direct current signal of level corresponding to the amplitude of the audio frequency output of the detector 36 to the I.F. amplifier 34 (and, possibly, also to the I.F. amplifier 28) to control the gain of the latter for providing an audio output intensity of generally the same level from received signals of strengths varying within certain limits.
The audio frequency output from the detector 36 of the tuner 20 is delivered concurrently, by connections as at 41, to each of the three modules 42, 44 and 46, which are commonly referred to as "light circuits". Each of the modules 42, 44 and 46 conventionally includes audio frequency filtering input means for passing only audio frequencies of a particular frequency corresponding to one of the modulation frequencies used to identify the three marker beacons (3000 Hz for the inner beacon and module 42, 1300 Hz for the middle beacon and module 44, and 400 Hz for the outer beacon and module 46), together with an electric indicating lamp and driver amplifier means for illuminating the lamp in response to passage by the associated filter means of audio frequency signals of the frequency to be indicated by the lamp of that module 42, 44 or 46. A test circuit 48 having a normally open manual switch 49 in series therewith is typically provided for checking the integrity of the lamps in all of the modules 42, 44 and 46 by momentarily applying an appropriate voltage from any suitable source thereof as at 47 (or a ground connection, depending upon the internal circuitry details of the modules 42, 44 and 46) to all of the lamps.
The audio frequency output from the detector 36 is also delivered to the audio portion 50 of the receiver system via a connection as at 51. Prior marker beacon receivers have typically employed some conventional type of audio amplifier 52 for receiving and amplifying the audio frequency output from the detector 36, with the output from the amplifier then being fed to a loudspeaker (or headphones) 54 via a connection circuit as at 56 having a normally closed manual switch 57 in series therewith. The switch 57 is typically part of a switch panel utilized for selectively connecting any one or more of the various instruments on the aircraft having an audio frequency output with the loudspeaker or headphones 54. It will also be understood that the switch 57 is the one previously referred to as commonly used by pilots to disconnect the marker beacon receiver system from the speaker or phones 54, once the entry of the aircraft into the reception zone of the outer or middle markers has been indicated by the system.
It remains in connection with the typical prior system depicted in FIG. 1 to consider the manner in which the previously mentioned manual control over receiver sensitivity has been conventionally implemented. The manual switch for such purpose is shown at 61 and will be seen to have a "HI" position represented by the contact 63 and a "LO" position represented by the contact 65. The terminal 67 represents any suitable source of direct current potential of level appropriate for "fooling" the I.F. amplifier(s) 34 (and 28) into "thinking" that the output of the detector 36 is at a relatively high level and that the detector 36 is sending an automatic gain control signal of correspondingly high level back to the intermediate frequency section of the receiver for causing the latter to amplify with a lesser level of gain than would otherwise be appropriate for the level of signals actually being handled. The available connection from the terminal 67 through the contact 65 of the switch 61 proceeds through a conductor 62 coupled with the direct current portion of the A.G.C. circuit 38 of the tuner 20. As intended to be used, the pilot sets the switch 61 to the "HI" position during approach to the outer marker beacon; then, when the system indicates that the aircraft has entered the reception area for the outer beacon, the pilot manually sets the switch 61 to the "LO" position for lowered sensitivity of the receiver until the pilot feels the aircraft should have passed over the outer beacon; then the pilot should restore the switch 61 to the "HI" position for higher sensitivity in detecting entry of the aircraft into the reception area for the middle beacon; with such sequence of manual switch resettings then being repeated as the aircraft traverses the reception area for the middle beacon and approaches the inner beacon. Aside from the apparent burden and distractions of the pilot associated with such conventional manual switching approach, certain more subtle disadvantages also accrue from employing a "false" A.G.C. signal to emulate the effects of true radio frequency sensitivity control. For example, with the prior approach, the ultimate sensitivity of the tuner 20 can be influenced not only by the "false" A.G.C. signal, but also by the real A.G.C. signal derived from the intensity of the signals actually being received at any given time, which makes it difficult if not impossible to accurately adjust conventional marker beacon receiver systems for lowered sensitivity operation at a known predetermined level of radio frequency signal strength, as is most desirable. Another disadvantage of the conventional "false" A.G.C. signal approach is that the extent of reduction in sensitivity brought about by setting the switch 61 to the "LO" position is critically dependent upon the level of the voltage actually provided at the terminal 67, that is, upon an analog type input (as contrasted with true switching of circuitry whose operation depends upon the connection or disconnection of components of known substantially constant value). Also, if the voltage represented by the terminal 67 is derived from a source at all remote from the switch 61, the length of the conductor 62 may subject the sensitivity control function to unpredictable effects of noise and electrical transients. It is noted that the lamp test switch 47 is frequently combined with the switch 61 as a third position of the latter, in manner that will be apparent.